When streetporting a 12a for N/A or forced induction, how much does one take from the exhaust port? I see very little information about porting other than the intake port. I have read only once that a "D" port reduces overlap and better flow, is this true?

 

Porting the exhaust for NA is a different shape than for a turbo is my understanding.

I don't recall the shapes involved. Maybe someone with clue can fill us in.

 

Look to pineapple racing. They tend to be proud of their exhaust port templates.

As for me, I just copy a T2 in timing (just have to go up). Or go up until it hits the sleeve. For mild ports, I like to only go up 2mm and recreate the bevel. I don't go down because these US-spec 12A ports already open at T2 spec. So they're already fine for open timing. They just close far too early for my taste. Compare. You'll see.